I dropped batgirl after issue 4. Supergirl has been such a suprise that I went back to buy the old volume once i ran out of the New52 series just to have more, even tho theres no relation between them!
issue 7 and 12 are good jumping on points depending on availability. The first arc will read alot better in the tpb which is only 15ish bucks. The first arc is decompressed, but after that it smooths out like a normal monthly comic.